Method and installation of filtration with compressible discs

ABSTRACT

A filter assembly is formed by arranging a large number of substantially annular, thin filter elements disposed around a radially pervious and hollow core so that a major portion of the core is surrounded by the filter elements. Slots between the filter elements allow passage of the liquid to be filtered. The filter assembly is enclosed in a closed vessel with the interior of the core being in communication with an outlet line. An initial axial pressure is established on the filter elements by a spring which acts on an end support engaging the end one of the filter elements. The axial pressure on the filter elements is controlled by controlling pressure in the closed vessel and thus the size of the slots is controlled to establish the desired filtering degree of the filter assembly.

This application is a continuation of application Ser. No. 464,886,filed Feb. 8, 1983, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method of arranging filters of thetype comprising a large number of substantially annular, thin filterelements on a radially pervious and hollow core, thereby forming slotsbetween adjacent elements to allow passage of the liquid to be filtered,and establishing a pressure difference between the space outside thefilter elements and the space within the core to permit the liquidtransport through the slots.

The invention also relates to an installation for carrying the methodinto effect, comprising a closed vessel and at least one filter assemblyprojecting into the vessel and consisting of a radially pervious orperforated and hollow core closed at one end and connected at the otherend with a space situated outside the vessel, and annular thin filterelements on the core and disposed between end supports of which at leastone is adjustable along the core.

In filters of the type referred to the filter elements often consist ofpaper or paper-like material and in such filters the applicationpressure between the filter elements is controlled by a screw means, aspring or the like which is tightened to a certain value. The liquid tobe filtered is allowed to pass radially through the filter element pile,and the purified liquid can be collected on the remote side and theimpurities remain on the opposite side. For cleaning of the filter,liquid is allowed to pass in the reverse direction. In conventionalfilters the compression of the filter elements cannot be pushed too farsince these would thereby be permanently deformed, which would graduallyalter the filtering capacity.

It has been found that the differential pressure can have a negativeinfluence on the function of the filter because the pressure in asurrounding vessel will have a compressive effect on the filter pilewhereby the application between the filter elements will be so heavythat the throughflow will be entirely interrupted under unfavorableconditions. It is interesting to note that the compression becomesself-increasing since, as the slots between the filter elementsdecrease, the throughflow decreases and the differential pressureincreases. To remedy this it has been proposed in French PatentSpecification No. 1,343,366 to provide a compression-preventing fluidchamber communicating with the space inside the filter elements andadapted to prevent the pressure in the surrounding vessel from actingupon the filter elements in a compressive sense. The spring-biased endplate engaging with the free end of the filter elements is balanced withthe aid of the pressure of the outgoing fluid the pressure of which actson both sides of the end plate which is displaceable like the piston ina cylinder. Therefore the known filter is not influenced by thedifferential pressure in the axial sense. The degree of compression isentirely controlled by the spring which pulls the lower end platetowards the fixed end plate at the outlet opening.

The present invention is based on the knowledge that the differentialpressure can act upon the compression of the filter elements, and theobject of the invention is to provide a means for utilizing thisknowledge in order to adjust by a simple operation the filtration degreeof the filter, i.e. vary the separating capacity of the filter upwardsas well as downwards and permit filtration with separation of very smallparticles.

The essential characteristic of the method of the invention resides in:enclosing in a closed vessel the core surrounded by a plurality ofannular filter elements; connecting the interior of the core with anoutlet line or the like; establishing an initial application of axialpressure on the filter elements by a spring or other means which isadapted to act upon an end support engaging an end one of the filterelements; and adjusting the application of axial pressure on the filterelements to control the size of the slots to establish the desiredfiltering degree by adjusting the differential pressure, i.e. thedifference between the higher pressure in the closed vessel and thepressure in the interior of the core, in such a way that the pluralityof filter elements will be axially compressed.

The essential characteristic of the installation of the inventionresides in a plurality of filter elements consisting of a material whichis recurrently compressible and substantially incapable of absorbing thefiltration liquid, which plurality of filter elements surround apervious hollow core, a spring element adapted to act upon adisplaceable end support engaging the end one of the filter elements toestablish an initial axial pressure on the plurality of filter elementsand a vessel containing the filter assembly of filter elementssurrounding the pervious core. The vessel is in the form of a pressurevessel to permit variable pressure buildup across the filter assembly sothat the size of the filter slots may be varied in response to thevariation in axial compression of the filter elements.

The invention will be described in more detail below with reference tothe accompanying drawing in which:

FIG. 1 schematically illustrates the buildup of an installation adaptedto the method of the invention; and

FIG. 2 is a partially sectional view showing on a larger scale a filterin such an installation.

In the drawing, 1 designates a tank containing liquid to be filtered. Asuction line 2 is coupled to a pump 3 provided with a pressureregulating means 4. The pressure line 5 of the pump leads to a pressureresistant vessel 6 in which the filter assemblies 13 are contained. Thevessel has a partition dividing up the vessel into a section 7 forunfiltered liquid and a section 8 for filtered liquid. A conduit forfiltered liquid leads from the section 8, via a control valve box 9, tothe station of use or storage. Cleaning medium can be passed in areverse direction the back way to the section 8 via the control valvebox 9 to carry out back flushing of the filters. A conduit 10 forseparated sludge extends from the vessel section 7 via the control valvebox 9 to a sludge container 11.

A large number of filter assemblies 13 are attached to the partition 12of the vessel 6. The filter assemblies each comprise a core 14 which ispervious or perforated adjacent to the partition 12 and is closed at thelower end. The interior of the respective core communicates with thespace 8 situated above the partition 12. Provided adjacent the upper endof the core is an external flange or abutment surface 15. A large numberof annular filter elements 16 surround the outside of the core. An endstop 17, which is displaceable relative to the core, bears against thelower end one of the filter elements and a compression spring 18 actingon the stop 17 is clamped against an abutment means 19 which isadjustable in the longitudinal direction of the core. The filterelements 16 are ring-shaped and consist of a thin material of such anature that it will yield to compression but will resume its initialshape as the compression ceases. Thus the material is not permanentlydeformable but compressible to a certain extent. A plastics materialwould seem to be suitable whereas paper-based material, which must beconsidered predominant in the field of filters today, does not have thedesired properties. Another essential requirement is that the materialmust not absorb liquid that makes it swell.

Since the pressure outside the filter is higher than the pressure in theinterior of the core 14 the liquid during filtration will be carriedthrough the slot-shaped interstices between the filter elements 16 andpass through the pores of the core 14 to exit through the interior ofthe core to the space 8 situated at the top. Impurities will remain onthe outside of the filter elements and can be removed therefrom by backflushing, i.e. allowing pure liquid to pass in opposite directionthrough the slots.

The filter can be made adjustable to fit different particle sizesbecause the material used for making the filter elements is homogeneousand it substantially resumes its shape after compression.

Such an adjustment is obtained by regulation of the differentialpressure, i.e. the pressure difference between the vessel space orsection 7 and the pressure in the interior of the cores. The presence ofa differential pressure is decisive for the function of the filter,which besides also applies to conventional paper filters.

The novelty resides in the feature that it is possible, due to thematerial used for the filter elements, to make the pressure in thevessel section 7 control the degree of compression of the filterelements, since the vessel pressure acts on the entire filter assemblyand tends to axially compress the plurality of filter elements. Thefilter elements are not substantially compressible in the radial sensebut are compressible in the axial sense. Therefore, an increase of thepressure in the vessel section 8 leads to an increased axial compressionof the filter elements and consequently to a reduction of the "height"of the slots.

To enable a pressure-controlled regulation of the degree of compressionof the filter elements it must first be possible to build up adifferential pressure. As long as the filter elements are looselyapplied on each other the vessel pressure cannot be utilized asindicated but first an initial compression must take place. This iseffected by means of the spring 18 which acts against the displaceableend stop 17. By application of the spring the differential pressure isadjusted so as to be in the order of 2 kp/cm². The filter element pileoffers such a high throughflow resistance that the pressure in thevessel space can be driven up to about 4-5 kp/cm² with the aid of thepump 3. By using a filter device in the indicated way it will bepossible to filter off particles of a size as small as 0.25-0.50 micronsand a further development would seem to make it quite possible to filteroff even amoebae, for instance for the purification of drinking-water.

In addition to purification of drinking-water and of water from sewagetreatment plants the filter installation can be utilized for thepurification of technical water, e.g. sealing water for pumps and spraywater for the paper and cellulose industry.

The invention should not be considered restricted to that describedabove and shown in the drawing but may be modified in various wayswithin the scope of the appended claims.

What we claim and desire to secure by Letters Patent is:
 1. A method ofcontrolling the size of material filtered from a liquid comprising thesteps of:providing a filter assembly including a pressure vessel, aplurality of filter discs placed around a pervious hollow core, the corebeing located within the vessel, the filter discs being axiallydisplaceable about the core, and the individual filter discs having anoriginal height, and being resiliently compressible along the axis ofthe core; applying an initial axially compression to the filter discs;passing the liquid through the filter assembly; axially compressing theplurality of filter discs; and reducing the original height of theindividual filter discs during the filter cycle; whereby the size of thematerials filtered from the liquid is decreased as the compression ofthe plurality of filter discs increases.
 2. The method according toclaim 1 further comprising the step of:connecting the interior of thefilter assembly with an outlet line.
 3. The method according to claim 1further comprising the step of:connecting the pressure vessel with aninlet line.
 4. An apparatus for filtering a liquid in which the size ofthe material filtered is controlled which comprises:a pressure vesselwhich has an inlet; a hollow pervious core, an end of the core being anoutlet; a plurality of filter elements which are stacked around thecore, the elements being compressible along the axis of the core, theindividual elements having an axial height, the filter elements beingmade of an elastic material, whereby the axial height is reduced byfluid pressure; and means for initially compressing the filter elementsin the axial direction.
 5. The apparatus according to claim 4 whereinthe filter elements are annular and disc-shaped.
 6. The apparatusaccording to claim 4 wherein the means for initially compressingcomprises a spring means adapted to act upon one end of the plurality offilter elements.
 7. The apparatus according to claim 4 wherein thefilter elements are made of plastic.
 8. The apparatus according to claim4 wherein the filter elements are non-absorbent.
 9. An apparatus forfiltering a liquid in which the size of the material filtered iscontrolled, which comprises:a pressure vessel having an inlet; a hollowpervious core, one end of the core being closed and the other end beingopen, the open end being in communication with an outlet; a plurality ofdisc-shaped and annular filter elements which are stacked around thecore, the elements being compressible along the axis of the core, theindividual elements having an axial height, the elements being made ofan elastic material, whereby the axial height is reduced by fluidpressure; and means for initially compressing the elements in the axialdirection, the means for initially compressing includes a spring meansadapted to act upon one end of the plurality of filter elements.